I second what Sosso said - there's no trick to keeping mass on a vegan diet any more than there is on an omni diet. Plenty of calories and lots of protein, it's that easy (I have been up to just under 130kg in years past, now holding tight around 108-110kg).

Theres's nothing magical in meat or dairy - get good quality food and don't miss out on the protein, train hard, and it all works out

"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous